The present invention relates to an apparatus and method for determining with accuracy the liquid water content of ambient air, particularly in relation to air flows across air vehicles or other structures. The accurate and timely measurement of liquid water content (LWC) permits prompt signalling for activating deicing systems, and also permits sensing atmospheric conditions for reporting or research purposes.
Unheated bodies exposed to airflow laden with supercooled water droplets will typically accrete ice as the droplets impact the body and freeze. Icing is particularly a problem with air vehicles. Determining when ice is starting to form or predicting when it will form is important in aircraft management of deicing equipment including heaters, which can consume huge amounts of power. When the air temperature is cold enough, 100% of the droplets carried in the airflow will freeze. If the temperature warms or airflow is increased, the energy balance relationship is altered. A critical liquid water content (LWC) is reached where not all of the impinging supercooled water droplets freeze. This critical LWC is defined as the Ludlam Limit. The Ludlam Limit is described in an article by F. H. Ludlam entitled The Heat Economy of a Rimed Cylinder. Quart. J. Roy. Met. Soc., Vol. 77, 1951, pp. 663-666. Additional descriptions of the problem are in articles by B. L. Messinger, entitled Equilibrium Temperature of an Unheated Icing Surface as a Function of Air Speed, Journal of the Aeronautical Sciences, January 1953, and a further article entitled An Appraisal of The Single Rotating Cylinder Method of Liquid Water Content Measurement, by J. R. Stallbrass, Report—Low Temperature Laboratory No. LTR-LT-92, National Research Council, Canada, 1978.
It has been shown that if the LWC increases above the Ludlam Limit, the accretion characteristics in theory remain unchanged, because excess water simply blows off or runs off, rather than freezing. Thus, present systems for determining liquid water content based on ice accretion suffer degraded accuracy above the Ludlam Limit. The Ludlam Limit for a given temperature and airflow is the liquid water content above which not all of the water freezes on impact with an accreting surface.
Accretion based ice detectors are frequently designed with probes that permit ice build up to a set mass, perhaps taking 30 to 60 seconds depending on conditions, at which time the presence of ice is enunciated or indicated, and a probe heater energized to melt the ice. Such ice detectors are well known in the art, and many depend upon a vibrating sensor or probe, with a frequency sensitive circuit set to determine frequency changes caused by ice accreting on the detector probe.
LWC can be roughly determined by monitoring a signal proportional to the probe icing rate, which again can be determined with existing circuitry, but accuracy degrades rapidly if the LWC is above the Ludlam Limit, because a portion of the impinging water never freezes. In such cases the actual LWC will be under reported, with the Ludlam Limit LWC being the maximum that will be reported. Even though the droplet cloud may contain additional liquid water, there will be no indication from such an ice detector that there is additional liquid water in the air flow. Thus, the prior art devices will not discern the actual liquid water content when the Ludlam Limit has been exceeded.